Oil is stored inside a device main body of a hydraulic pump that supplies oil to a hydraulic actuator such as a hydraulic cylinder. Oil lubricates and cools a rotation supporting portion such as a bearing and a sliding portion of mechanical parts. In addition, a filter element is housed inside the device main body of an oil filter arranged in a hydraulic circuit. The filter element filters and then feeds the oil stored inside. In the above-described hydraulic pump, foreign matter such as abrasion powder generated at the rotation supporting portion and the sliding portion may be mixed in the oil. Foreign matter mixed in the oil may cause problems such as uneven wear and damage on the rotation supporting portion and the sliding portion. Further, in the oil filter, there is a possibility that foreign matter mixed in while passing through the hydraulic circuit may clog the filter. Therefore, in this type of hydraulic device, how to easily check the state of oil from the outside of the device main body is an important subject. A conventional hydraulic device is also provided with a window hole formed in the device main body and a lens attached to the window hole (see, for example, Patent Literature 1).